I found this posted on the guns and ammo site,
http://archives.gunsandammo.com/content/shock-value.
Hornady's SSTs differ from other plastic-tipped rifle bullets in that they were designed from the ground up to be a big-game bullet, not a varmint pill. For starters, the copper jackets used on the SST bullets is .0010 thicker than the jackets used on Hornady's V-Max varmint bullets, and have a tighter grain structure. When the jacket cup is fully drawn, the jacket itself is 19 percent thicker, which aids in weight retention and assists in penetration. In addition, the lead core of the SST is a harder alloy, featuring a five-percent antimony content. These premium ingredients, combined with Hornady's Interlock construction helps hold the heavier and tougher jacket and core together until reaching the vitals on big game
This was a comment on a forum about the FTX.
These bullets look almost exact to a Hornady SST bullet. They are not. The SST'S have a hard plastic tip. The FTX have a soft plastic tip.
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And this was on the Brownell site. It is not as detailed as the information on the SST with regard to jacket thickness, composition, etc.
The Hornady FTX bullets have the patented Flex Tip design rendering them safe in tubular magazines found on most lever action guns. The Flex Tip technology delivers excellent terminal performance and controlled expansion across a broad range of velocities. Upon impact, the soft tip compresses into the bullet, initiating immediate expansion that transfers energy to the target for quick kills. The famous Interlock feature, locks the core and jacket, eliminating core and jacket expansion. The Hornady FTX delivers the flattest trajectories
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It sounds to me that the difference is in the terminal performance (expansion). I have loaded and shot a number of different blems from Midway with no problems. Some were 250 gr and some were 225. All had the red tip, but some were listed as being for muzzle loaders with a sabot in the description on Midway's site - all were .452, some of the 225 had cannelures, which the 250's do not. No problems.